Glucagon-like peptide 1 (GLP1) is a hormone that controls energy homeostasis, insulin release, and feeding behavior. GLP1 activates a complex neuroendocrine axis that coordinates systemic behavioral and metabolic responses to nutrient intake, with GLP1 receptor (GLP1R) expressed in many locations, including the vagus nerve, brainstem, and hypothalamus. The roles of some GLP1-responsive neurons remain poorly defined, in part due to a lack of appropriate genetic tools. Understanding the roles of different GLP1-responsive neurons is imperative, as therapeutic strategies that involve mimicry or stabilization of GLP1 provide clinically important methods for control of diabetes and potentially other metabolic disorders. GLP1R is expressed in a cohort of vagal sensory neurons, although the roles of vagal GLP1R neurons in feeding behavior, metabolism, and nausea are highly controversial. Recent studies indicate that vagal GLP1R is not required for the effects of GLP1R agonists on body weight or diabetes resolution, raising basic questions about (1) where vagal GLP1R neurons project in the body and brain, (2) what they detect, and (3) what physiological responses they evoke, all of which are unknown. In preliminary data, we used a molecular and genetic approach to deconstruct the sensory vagus nerve into cellular components. We generated a collection of 'ires-Cre' knock-in mice, including Glp1r-ires-Cre mice, and adapted powerful genetic techniques for connectivity mapping, in vivo imaging, and optogenetic control of neural activity in the vagus nerve. Using these approaches, we identified two subtypes of vagal afferents that innervate the lung, and exert powerful and opposing effects on breathing (Cell, 2015), and here, will use related techniques to study the sensory biology of vagal GLP1R neurons. In Aim 1, we developed a strategy for introducing genetic tracers into vagal sensory neurons by adeno-associated virus infection, and will use this technique to map the projections of vagal GLP1R neurons in peripheral organs and the brainstem. In Aim 2, we developed a novel in vivo imaging paradigm in vagal ganglia that involves a genetically encoded calcium indicator, and will use this technique to query the specific response properties of vagal GLP1R neurons. In Aim 3, we will selectively activate or eliminate vagal GLP1R neurons using genetic approaches, and determine the impact on feeding behavior, metabolism, nausea, and other aspects of autonomic physiology. Together, these studies will provide needed information about the cell biology of vagal GLP1R neurons. Charting GLP1- responsive circuits at a cellular level will help reveal how GLP1 evokes diverse physiological responses in health and disease, and may provide an important foundation for future therapy development.